Versatile X-Ray Beam Scanner

ABSTRACT

Apparatus for interrupting and/or scanning a beam of penetrating radiation, such as for purposes of inspecting contents of a container. A source, such as an x-ray tube, generates a fan beam of radiation effectively emanating from a source axis, with the width of the fan beam collimated by a width collimator, such as a clamshell collimator. An angular collimator, stationary during the course of scanning, limits the extent of the scan, and a multi-aperture unit, such as a hoop, or a nested pair of hoops, is rotated about a central axis, and structured in such a manner that the total beam fluence incident on a target is conserved for different fields of view of the beam on the target. The central axis of hoop rotation need not coincide with the source axis.

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 13/280,941, filed Oct. 25, 2011, and, like thatapplication, claims the priority of U.S. Provisional Application Ser.No. 61/407,113, filed Oct. 27, 2010, and of U.S. Provisional ApplicationSer. No. 61/533,407, filed Sep. 12, 2011, all three of which priorapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to methods and apparatus for interrupting,steering and/or varying the spatial sweep and resolution of a beam ofradiation, and, more particularly, a beam used for x-ray inspection.

BACKGROUND ART

One application of x-ray backscatter technology is that of x-rayinspection, as employed, for example, in a portal through which avehicle passes, or in a system mounted inside a vehicle for inspectingtargets outside the vehicle. In such a system, an x-ray beam scans thetarget and detectors measure the intensity of backscattered radiation asthe inspection vehicle and target pass each other. During inspectionthat images backscattered x-rays, it would be desirable for the operatorto be able to control the x-ray beam's viewing angle, viewing direction,beam resolution and filtration.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In accordance with embodiments of the invention, methods and apparatusare provided for scanning a beam of radiation with respect to a target.In certain embodiments, a scanning apparatus is provided for scanning abeam in a single dimensional scan. The apparatus has a source ofradiation for generating a fan beam of radiation effectively emanatingfrom a source axis and characterized by a width. The source also has anangle selector, stationary during the course of scanning, for limitingthe angular extent of the scan, and a multi-aperture unit rotatableabout a central axis in such a manner that beam fluence incident on atarget is conserved for different fields of view of the beam on thetarget. In some embodiments, there may also be an inner or an outerwidth collimator for collimating the width of the fan beam.

In accordance with alternate embodiments of the present invention, themulti-aperture unit may include rings of apertures spaced laterallyalong the central axis in such a manner that relative axial motion ofthe multi-aperture unit relative to the x-ray beam plane places a ringof apertures in the beam that is collimated by a corresponding openingangle in the angle selector.

In accordance with other embodiments of the invention, the angleselector may include a slot of continuously variable opening. Thecentral axis may be substantially coincident with the source axis, oreither forward-offset of rearward-offset relative to the central axis.The multi-aperture unit may include rectangular slots.

In accordance with yet other embodiments of the invention, the scanningapparatus may also have a collimator, which may either be an “innercollimator” or an “outer collimator,” each of which is defined below.The collimator may limit one or both of the width of the beam or theangular extent of the scan. In the former case, the collimator isreferred to as a width collimator. An inner width collimator may includetwo or more slots of different widths. The width collimator, in someembodiments, may be fixed in width during the course of scanning thebeam.

In accordance with further embodiments of the present invention, theangle selector may include a plurality of discrete slots, and also ashutter position.

In accordance with more embodiments of the present invention, the sourceof radiation may be an x-ray tube, and may be a source of radiation of atype generating a fan beam exceeding 60° in opening angle. The scanningapparatus may also be coupled to a platform, and may have an enclosingconveyance for conveying the scanning apparatus past an inspectiontarget. The scanning apparatus may be coupled to a platform inconjunction with at least one further scanning apparatus.

In accordance with alternate embodiments of the invention, the scanningapparatus may have a filter disposed within the beam for changing theenergy distribution of the beam and/or for governing a dose of radiationincident on a target or portion of a target. The filter may be disposedon a filter tube, which may be adapted for selecting insertion of aplurality of filters and may include a beam shutter.

The multi-aperture unit may include two nested, multi-aperturecollimators, and may include an inner multi-aperture hoop made ofmaterial opaque to the beam. The multi-aperture unit may additionallyinclude rings of apertures spaced laterally along the tube axis in sucha manner that relative axial motion of the inner multi-aperture hooprelative to the beam plane places a ring of apertures in the beam thatis collimated by a corresponding opening angle in the angle selector.The multi-aperture unit may also include an outer multi-aperture hooprotatable in registration with the inner multi-aperture hoop. The outermulti-aperture hoop may include a plurality of apertures configured ashorizontal slots in such a manner as to define a minimum size of emittedpencil beams along a sweep direct of the beams. Where there are twomulti-aperture hoops, the inner and outer multi-aperture hoops may bemechanically integral. There may be an outer variable width collimatorfor defining a width of the beam that enters or exits the outermulti-aperture hoop.

In other embodiments of the scanning apparatus, radiation may be emittedthrough a plurality of apertures at different angles with respect to thetarget, such that pencil beams of penetrating radiation sweep inalternation through the target in such a manner as to provide astereoscopic view of an interior volume of the target. The scanningapparatus may have a rotation assembly adapted to provide for rotationof the source of radiation about the source axis, such as to reduceabsorption of emitted radiation in the source anode, an effect referredto as the “heel” effect. The multi-aperture unit may includesubstantially rectangular through-holes.

In accordance with alternate embodiments of the invention, a chopper isprovided for interrupting a beam of particles, wherein the chopper hasan obscuring element substantially opaque to passage of the particles inthe propagation direction, and at least one through-hole in theobscuring element adapted for passage through the obscuring element ofparticles in the propagation direction, where the through-hole ischaracterized by a tapered dimension in a plane transverse to thepropagation direction. Finally, the chopper has an actuator for movingthe obscuring element in such a manner as to cause at least a portion ofthe beam of particles to traverse the at least one through hole on aperiodic basis.

The through-hole of the chopper maybe substantially conical orsubstantially biconical. The particles in the chopped beam may bemassless, including electromagnetic radiation over a specified range ofwavelengths.

In other alternate embodiments, a chopper is provided for interrupting abeam of particles that has an obscuring element substantially opaque topassage of the particles in the propagation direction and at least onethrough-hole in the obscuring element adapted for passage through theobscuring element of particles in the propagation direction, thethrough-hole characterized by a rectangular cross section in a planetransverse to the propagation direction.

In yet other alternate embodiments, a collimator is provided fornarrowing a beam of particles, where the collimator has an obscuringelement substantially opaque to passage of the particles in thepropagation direction, and a gap in the obscuring element where thewidth of the gap varies as a function of distance along the longdimension relative to an edge of the gap. The gap may be fixed oradapted to be varied in at least one of the long and narrow dimensions.

In accordance with another embodiment of the invention, a beam choppingassembly is provided that has a rotating element including at least onethrough-hole of rectangular cross-section.

In yet other embodiments of the present invention, a method is providedfor inspecting an object based on the transmission of x-rays through anobject. The method has steps of:

-   -   a. generating a fan beam of radiation;    -   b. collimating the width of the fan beam with a collimator that        is stationary during the course of a scan of the object, the        scan characterized by an extent;    -   c. limiting the extent of the scan with an angle selector; and    -   d. varying a field of view of the beam on the object by means of        a multi-aperture unit rotatable about a central axis in such a        manner that beam fluence incident on a target is the same per        revolution for all selected scan angles.

The method may also include varying a direction of the scan, and thestep of varying a field of view may include varying from a primary rapidscan to a secondary high-resolution scan of a suspect area.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1 shows an exploded view of major components of a basic unit inaccordance with one embodiment of a versatile x-ray beam scanner;

FIG. 2 depicts a version of a slot inner width collimator used tocontrol the width of a fan beam from an x-ray tube, in accordance withan embodiment of the present invention;

FIG. 3A shows a version of the angle selector that controls the angle ofthe fan beam from the x-ray tube, in accordance with an embodiment ofthe present invention, while FIGS. 3B-3E show views of a continuouslyvariable angle selector in accordance with a further embodiment of thepresent invention;

FIG. 4 shows an inner multi-slot aperture unit that rotates to createthe scanning pencil beam, in accordance with an embodiment of thepresent invention;

FIG. 5 shows an assembly view of a basic versatile beam scanner, inaccordance with an embodiment of the present invention;

FIG. 6 shows a flattened depiction of the inner multi-slot apertureunit, more particularly showing an arrangement of slots to obtain 90°-,45°-, 30°- or 15°-views, in accordance with a preferred embodiment ofthe present invention;

FIG. 7 is an exploded view of the full version of a versatile beamscanner showing the addition of a filter wheel, and the outermulti-apetrture hoop with slot through-holes and an outer widthcollimator with variable jaw spacing, in accordance with an embodimentof the present invention;

FIGS. 8A and 8B are front and perspective views of one embodiment of acollimator of the present invention;

FIG. 9 shows an assembly view of a versatile beam scanner, in accordancewith an embodiment of the present invention; and

FIG. 10A is a cross-sectional depiction of an alternate embodiment ofthe invention in which the inner multi-aperture unit and outermulti-aperture hoop are rigidly coupled to form a bundt-cake scanner, inaccordance with an embodiment of the present invention. FIG. 10B shows aschematic view of elements of a versatile beam scanner, in accordancewith the embodiment of the present invention depicted in FIG. 7.

FIG. 11A shows a flattened depiction of the inner multi-aperture unit,with slots for 90°-, 45°-, 30°- or 15°-views, all slots of identicalheight, in accordance with an embodiment of the present invention. InFIG. 11B, an additional ring of half-height slots is added, and FIG. 10Cshows a slot pattern for obtaining two separate 15° views, both inaccordance with other embodiments of the present invention.

FIG. 12 is a schematic cross-sectional depiction of a versatile beamscanner, in accordance with the present invention, with an x-ray tube ina forward offset position.

FIG. 13 depicts the geometry of a rotating aperture hoop with an axisoffset with respect to an x-ray tube, in accordance with embodiments ofthe present invention.

FIG. 14 plots the dependence of the fan-beam angle, 2φ, on the ratioD/R, for forward and backward values of D, in accordance withembodiments of the present invention.

FIG. 15A-C are schematic cross-sections of an embodiment of theinvention in which an x-ray source may be rotated, from ahorizontal-pointing orientation in FIG. 15A to an orientation depressedby 52.5° shown in FIGS. 15B and 15C.

FIG. 16 is a perspective view of a rotatable basic unit including arotatable x-ray source, in accordance with embodiments of the presentinvention.

FIGS. 17A and 17B are perspective and cross-sectional views,respectively, of a biconical aperture (or through-hole) for beamchopping, in accordance with an embodiment of the present invention,while FIGS. 17C and 17D are perspective and cross-sectional views,respectively, of a conical aperture (or through-hole) for beam chopping,in accordance with another embodiment of the present invention.

FIG. 18 depicts one example of an application of embodiments of thepresent invention, wherein a beam is swept in conjunction withbackscatter inspection of a target object.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Definitions

As used herein, and in any appended claims, the following terms shallhave the meanings indicated unless the context requires otherwise.

“Beam resolution,” as used herein, shall refer to the product of avertical resolution and a horizontal resolution. “Vertical” refers tothe plane containing the swept pencil beam described herein, i.e., aplane perpendicular to the axis of rotation of the hoop describedherein. The terms “horizontal” and “width” refer herein to the “axial”direction, which is to say, a direction parallel to the axis of rotationof the hoop(s) described herein.

“Resolution,” in either of the foregoing vertical or horizontal cases,refers to the height (for instance, in angular measure, such as degrees,or minutes of arc, etc.) of the pencil beam when stationary on astationary target, and the term assumes a point-like origin of the x-raybeam. Similarly, the areal beam resolution has units of square degreesor steradians, etc. Alternatively, resolution may be quoted in terms ofa point spread function (PSF) at a specified distance from a definingaperture.

The “zoom angle” is the angular extent of the scanning x-ray beam in thevertical direction, designated by numeral 15 in FIG. 1.

The term “commensurate,” as applied to angular intervals, refers tointervals related by whole number ratios, such that rotational cycles ofdistinct components repeat after a complete revolution of one component.

The term “fluence,” unless otherwise noted, is used herein, and in theappended claims, to mean the total integrated x-ray intensity in thechosen scan angle, for each revolution of the chopper wheel. Fluence issometimes referred to as “flux,” although “flux” may sometimes haveother meanings.

The term “areal density” as applied to an x-ray beam, shall refer toinstantaneous x-ray intensity per unit area delivered to a region of thetarget.

As used herein and in any appended claims, a collimator shall bereferred to as “inner” if it lies closer to a source of radiation thanany hoop of apertures rotating about an axis coinciding with, orparallel to, the axis of the source of radiation. A collimator shall bereferred to as “outer” if it is disposed further from a source ofradiation than a hoop of apertures rotating about an axis coincidingwith, or parallel to, the axis of the source of radiation.

Preferred embodiments of the present invention provide a versatile beamscanner (VBS) (or, “flexible beam former” (FBF)), which may,particularly, refer to a mechanism in which the intensity of x-rays on atarget increases inversely with the angular field of view on the target.While embodiments of the invention are described, herein, with referenceto x-rays derived from an x-ray source, it is to be understood thatvarious embodiments of the invention may advantageously be employed inthe context of other radiation, whether electromagnetic or relating tobeams of particles, and that all such embodiments are within the scopeof the present invention.

It should also be understood that embodiments of the present inventionmay be applied to the formation of images of x-rays transmitted througha target as well as to the formation of images of x-rays scattered fromthe target, or for any application where steering and focusing a beamsubject to conservation of beam fluence might be advantageous.

In particular, in various embodiments of the present invention, aversatile beam scanner may advantageously be mounted on a vehicle orconveyance of any sort, or on a portal inspecting moving objects.Moreover, multiple versatile beam scanners may be mounted on a singleportal or other platform, with beams temporally or spatially interleavedto preclude or reduce crosstalk.

The resolution of a beam on a target, where the beam is formed through acollimating hoop, is determined by the target's distance, the height ofthe collimation slots in the outermost hoop, and the width of thevariable width collimator that is adjacent, either directly inside ordirectly outside the outermost hoop. Methods, in accordance withembodiments of the present invention, provide for improving an image byimproving the vertical resolution of the scanning pencil beam, andproviding independent views with different vertical resolutions. Theseare discussed in detail, below.

In accordance with preferred embodiments of the present invention, theaxial (width) resolution is controlled with a variable collimator 180(shown in FIG. 7, and referred to herein as an outer width collimator).The angular (height resolution) is controlled by the integration time,and by two other parameters: the combination of wheel speed and scanangle, and a time constant associated with x-ray detection, namely thedecay time of a scintillation phosphor. Typically, the integration timeis set between 1 μs and 12 μs, with the number of resolved pixels in avertical scan determined by the scan angle and rotational speed. Forpurposes of example, a hoop rotation rate of 3600 rpm, with 6scans/revolution (as explained in detail below), and 500 pixels perscan, corresponds to ˜6 μs integration, and a beam resolution ofapproximately 0.1° per pixel.

Basic elements of a VBS may be separated into a first part—an innerscanner, described with reference to FIG. 1, and designated generally bynumeral 2, that is common to many embodiments, and a second part—anouter scanner 200 (shown in FIG. 7), that may be omitted for someapplications. In particular, for low-energy applications, preferredembodiments employ a single scanner, and, more particularly, a singleaperture ring, as discussed in detail, below. Also, for close objects,use of a single aperture ring, as described below, is preferred.

While, for purposes of explanation herein, the elements of a VBS aresummarized as a series of elements with increasing radii, it is to beunderstood that the order of the elements in the inner scanner can bevaried. Elements of the VBS may include:

-   -   a source 4 of penetrating radiation, such as an x-ray tube, that        emits a fan beam 8 of x-rays over a wide angle, preferably        greater than 60°, such as 120°, and in a plane (referred to,        herein, as the “vertical” plane) that is typically perpendicular        to the direction of vehicle and target passage;    -   a selectable filter 155, mounted in filter tube 150 (shown in        FIG. 7), for changing the energy distribution of the x-ray beam        or for adjusting the radiation dose delivered to a target or to        a portion of the target;    -   an inner width, or slot, collimator 14 and angle selector 34 in        the plane of the x-ray beam, made of material that is opaque to        the x-ray beam, that control the scan angle and scan direction;    -   a multi-aperture tube 50, made of material opaque to the x-rays,        which rotates through the fan beam created by the        slot-collimator to create a sweeping pencil beam;    -   an outer widthcollimator 180 (shown in FIG. 7), stationary        during scanning, having an adjustable jaw width 185 that        controls the horizontal width of the x-ray beam that inspects        the target; and    -   an outer multi-aperture hoop 170 (shown in FIG. 7) that rotates        in registration with the inner multi-aperture unit.

It is to be understood that the versatile beam scanner described hereinmay operate with a solitary hoop or ring of apertures. In that case itmay be advantageous to place a variable width collimator outside thehoop or ring. In the case where both an outer hoop and an inner ring areemployed, the beam-forming requirements of the outer hoop areadvantageously reduced, since the beam incident on the outer hoop isalready collimated to a pencil beam. Thus, x-ray opaque material needonly be provided around the apertures of the outer hoop 170.

One application of a versatile beam scanner, designated generally bynumeral 3, is depicted in FIG. 18, solely by way of example, and withoutlimitation. X-ray source 4 is mounted on an x-ray inspection vehicle180, providing transverse motion relative to a target of inspection 181.By operation of source 4 and scanner 3, x-ray beam 182 is scanned acrosstarget 181, and backscattered radiation 184 is detected by detectormodules 100, with one or more detector signals generated by detectormodules 100 subsequently converted by a processor 180 into an image ofcontents of target 181.

Referring to FIG. 1, the selectable widths of slot 22 (and 24) of slotcollimator 14 defines the width of fan beam 8, which is emitted fromx-ray tube 4 and effectively emanates at, or near, a source axis 6. Themaximum opening angle of fan beam 8 is the x-ray tube's beam angle; itdefines the maximum angular sweep 15 of the pencil beam. The openingangle for inspecting target (shown in FIG. 18) can be changed, either bythe operator, or by operation of processor 188 (shown in FIG. 18). Theopening angle may be changed in fixed steps commensurate with 360°, withthe maximum angle, as stated, limited by the x-ray tube's beam angle.The angle selector 34 can be rotated to change the direction of thesweep. Angle selector 34 typically remains fixed during the course ofscanning.

Angle selector 34 has rings of apertures 40 (best seen in FIG. 3A) thatdefine the angular extent of the scan of the pencil beam 70. Thecombination of the slot collimator 14 and the apertures 56 in in theaperture ring 50 defines the cross-section of pencil beam 70 (shown inFIG. 4). Each lateral ring of apertures 40 corresponds to one of thequantized opening angles of variable-slot collimator 14. When one of theopening angles of slot collimator 14 is chosen, angle selector 34 ismoved laterally to place the appropriate ring of apertures in the beam.The number of apertures in each ring is commensurate with 360°.Alternatively, angle selector 34 may provide for continuous variation ofopening angle from closure (as shown in FIG. 3B) to an opening of 120°(as shown in FIG. 3E), with other opening angles shown by way ofexample.

The zoom angle, i.e., the angular extent of the scanning x-ray beam, maybe determined by the lateral position of the spinning innermulti-aperture unit 50 and outer hoop 170. “Lateral,” as used herein,refers to a position along an axis parallel to the axis 6 about whichcomponents 50 and 170 rotate. In order to change that lateral position(and, thereby, the zoom angle), the offset of the plane of the fan beamis varied (in a step-wise fashion) with respect to the plane ofapertures that define the zoom angle. (The offset is relative; eitherthe beam or the aperture plane may be moved.) In a preferred embodimentof the invention, the aperture devices, which are rotating at highspeed, are not be translated, but, rather, the rest of the beam formingsystem is translated with respect to rotating aperture devices, however,it is to be understood that either configuration falls within the scopeof the present invention.

When the target (not shown) is distant from the inner scanner 2, theouter unit 200 may preferably be used to further define thecross-section of the pencil beam at the target. Referring now to FIG. 7,the outer unit 200 consists of a slot-collimator 180 (shown in FIG. 7)to refine the width of the scanning beam, and a rotating hoop 170 withapertures 175 to refine the height of the pencil beam 70. The apertures175 in the outer hoop 170 are equally-spaced, and their number is equalto the maximum number of apertures in a ring of the inner multi-aperturetube 50. The number is also commensurate with the number of apertures ineach of the rings of the inner beam-forming unit. The outer hoop islight-weight, thereby advantageously reducing its rotational moment ofinertia. The beam defining apertures are typically tungsten inserts.

The slotted outer width collimator 180 (shown in FIG. 7), withadjustable jaw width, controls the horizontal width of the x-ray beamthat inspects the target, and is stationary during scanning. The slotcollimator, 180, shown interior to the aperture ring 170, may also beexterior to it, within the scope of the present invention.

One novel and advantageous feature of embodiments of the presentinvention is the focusing feature. The decrease of the scan angle—inorder to focus on a portion of the target—results in a correspondingincrease in the beam intensity, since the number of slots illuminated bythe source per revolution of the hoop increases as the scan angledecreases. Thus, the resulting beam fluence on the target is the sameper revolution for all selected scan angles. This means that the arealdensity (defined above) of x-rays in a 15° view is six times greaterthan in a 90° view of the target. A further novel feature is theoperator's ability to change the cross-section of the scanning pencilbeam by moving the jaws of the fixed collimator 14, or the variablecollimator 180, to change the width of the image pixel, or changing theintegration time of the detected signal to change the height of theimage pixel. Yet another novel feature is the operator control of theviewing direction of the x-ray scan.

In accordance with certain embodiments of the present invention, angleselector 34 and/or aperture ring 50, and/or variable collimator 180 maybe selected automatically by processor 188 on the basis of the proximityof inspected target 181 (shown in FIG. 18), and the height or relativespeed of the inspection system and inspected target. One or more sensors186 (shown in FIG. 18) may be used to determine one or more of theforegoing parameters. Imaging data may also be used for that purpose.Similarly, filter 155 and collimator 180 may also be adjusted on thefly, such as to control a radiation dose on the basis of human occupancyof the inspected target, for example.

The flexible beam former, in accordance with the various embodimentstaught herein, may be advantageously applied to the formation of imagesof x-rays transmitted through a target or to the formation of images ofx-rays scattered from the target. It can be applied to a scan taken byrotating the scanning system. It can be implemented by manual changescarried out when the scanner is turned off, though the preferredembodiment is for changes carried out during the scan and evenautomatically in response to programmed instructions.

The versatility of the x-ray scanners taught herein allows the operatorto obtain the most effective inspection for targets at distances andrelative traversal speeds that can each vary over more than an order ofmagnitude.

Without loss of generality, the apparatus and methods described hereinmay be applied here to image formation of x-rays backscattered from atarget that moves perpendicularly at constant speed through the plane ofthe scanning pencil beam.

Embodiments of the invention, in several variants, are now describedwith reference to FIGS. 1 to 8. In a preferred embodiment, describedwith particular reference to FIGS. 1-7, a single beam of x-rays isproduced, under operator or automatic control, that scans the targetthrough selected field-of-view angles of 90°, 45°, 30°, or 15°, with achosen cross-section, at the target. The 90° opening is the normalposition; the three other openings provide 2×, 3× and 6× zooming. Ofcourse, it will be understood that the basic concepts described hereinmay readily be applied to applications that may involve a differentnumber of different scanning angles as well as different x-ray energies.The concepts can also be applied to the creation of beams that scan atdifferent inclination angles through the target.

Referring to FIG. 1, a scanning apparatus is designated generally bynumeral 2. An x-ray tube 4 produces a fan beam of x-rays 8 that isemitted perpendicular to the x-ray tube axis 6. An angle-defining unit10, which is stationary during a beam scan, intercepts the beam 8. Theangle-defining unit 10 defines the width and angle of the fan beam,either through operator control or automatically according to externalcriteria. In a preferred embodiment, the angle-defining unit 10 is avariable slot shown in a simplified version in FIGS. 3B-3E.Angle-defining unit 10 is opaque to the x-ray beam 8 except for thecontinuously-variable slot 41 (shown in FIG. 3C, by way of example),whose opening angle and pointing direction may be controlled by servomotors. FIG. 3B shows the slot closed, while FIGS. 3C-3E show openingangles of 15°, 60° and 120°, respectively.

It should be noted that alternate methods for obtaining the versatilityprovided by tubes 14 and 34 are within the scope of the presentinvention. Further versatility can be provided by rotating the entirex-ray producing unit including the x-ray tube itself, as furtherdescribed below.

Angle-defining tubes 14 and 34 can be rotated so that opaque sections ofboth tubes intercept the exiting beam without shutting down the x-raytube or the beam-forming wheels. Rotation of the unit 10 allows thesweeping beam to point in any directions inside the maximum fan beam 8from the x-ray tube. Further versatility in aiming the fan beam can beobtained by rotations of the entire x-ray generator. Angle selector 34,or another element, may serve as an x-ray shutter, whose power-offposition is closed, to shutter the x-ray beam to comply with safetyregulations. The shutter can be combined with other features such as thefilter changer. More particularly, filter tube 150 (shown in FIG. 7) mayhave multiple angular positions, one of which (such as its “parked”position) may include an x-ray-opaque element serving as a beam shutter.

Sweeping pencil beams 70 are formed by a tube 50 with apertures 56 (bestseen in FIG. 4) that rotates through the fan beam created by the innercollimators collectively labeled 10. Tube 50 is made of material opaqueto the x-rays. The height of apertures 56 together with the width ofslot 22 or 24 define the cross-section of pencil beam 70 that exits fromthe scanner 2.

In the preferred embodiment of tube 50, the apertures are slots 56rather than the traditional holes. The apertures of tube 50 and hoop 170may be slots in both cases. Slots 56 are arranged in a pattern that isdetermined by the maximum scan angle and the number of smaller scanangles in the design. The total number of slot apertures is commensuratewith 360°. The scan angles are also commensurate with 360°. FIG. 6 showsthe pattern in a depiction in which the multi-aperture tube 50 isstretched out as a flat ribbon 80. The aperture slots 84 are dark grayhorizontal bars, while the beam position is a light gray ribbon. Theslots are arranged in the 4-choice example above: 90°, 45°, 30°, and15°. Ribbon 80 has a four-fold repeat pattern of 6 slots, making a totalof 24 slots along the circumference. The slots are arranged so that eachof the 4 angular openings, 90°, 45°, 30° or 15°, can be placed in thebeam 70 by moving the tube 50 laterally.

Variable Beam Scanner for Distant Targets.

The basic unit 2 (shown in FIG. 1) has applications for inspectingtargets that are close enough to the beam-forming aperture for thescanning x-ray pencil beam to create a useful image. An x-ray inspectionsystem, mounted inside a vehicle, and used, for example, to imagetargets outside the vehicle, requires, in practice, an additional beamforming aperture to usefully inspect targets outside the vehicle.

As a rule of thumb, with many exceptions, the beam-forming aperture 175(in FIG. 7) should not be much further from the target than five timesthe distance from the x-ray tube's focal spot to the beam-formingaperture; the closer the better. The basic unit 2, shown in FIG. 1, can,in principle, be used for distant objects by making the diameter of themulti-aperture tube 50 as large as necessary. This approach can beuseful for low-energy x-ray beams that can be effectively shielded byrelatively light-weight hoops. For x-ray energies in the hundreds ofkeV, which require thick shields of high-Z material, a large radiusresults in a large rotational moment of inertia, which in turn limitsthe rotational speed of the beam scanner, and that in turn limits thespeed with which the inspection unit can scan the target.

The solution to the aforementioned difficulty is to use themulti-aperture tube 50, constructed of x-ray-opaque material, as aninitial collimator and add a light-weight, rotating large-diameter outerhoop 170, and another stationary outer width collimator 180 to refinethe cross section of the pencil beam. This concept is illustrated inFIGS. 7 and 8. Before describing these figures, the importance of thisapproach is further elaborated.

The rotational moment of inertia of a hoop is proportional to MR², whereM is the mass of the hoop and R is its radius. The mass M required toeffectively absorb an x-ray beam of a given energy is itselfapproximately proportional to the radius R since the thickness of theneeded absorber is approximately independent of radius. Thus therotational moment of inertia of the multi-aperture hoop is approximatelyproportional to the cube of the hoop's radius. Example: An 8″ OD tubemade of ½″ thick tungsten has a rotational moment of inertia that is 25times smaller that of a 24″ OD tube made of ½″ thick tungsten. (Thethicknesses correspond to 20 mean free paths (mfp) of absorption at 180keV, i.e. an attenuation of ˜10⁹.) Combining the smaller radius tungstentube with an outer hoop made almost entirely of light-weight materialresults in a significantly lower moment of inertia of the system, hencea higher maximum rotational speed.

FIG. 7 is an exploded view showing the elements of a preferredembodiment for distant targets. Each element is considered in turn.Basic unit 2 is the same as that shown in FIG. 1 except for the additionof an x-ray filter 150 in the form of a thin tube that surrounds x-raytube 4. An empty slot in one quadrant of the filter tube 150 allows thefull x-ray fan beam 8 to emerge. Filter tube 150 can be rotated so thatdifferent filters can intercept the fan beam to change the energydistribution or the deposited dose at the target, or to block anyemergent beam entirely. For example, a truck may be scanned with anautomatically inserted filter 155 to reduce the dose when the passengercompartment is being scanned. The variable filter tube may be omitted ifthe application does not require changing the energy distribution of thex-ray beam.

The maximum opening angle of the scanning beam is defined by the slotcollimator 14 with its discrete set of slots or the continuouslyvariable slot 41 shown in FIGS. 3B-3E, whose angular extent iscontrollable. As above, an inner aperture ring coarsely generates asquare flying spot by passing a slot (up to 24 slots per revolution inthe examples herein) across the fan-beam slit. After the beam passes outof the inner aperture ring 58, it travels until it encounters a pair ofjaws 180 that has an adjustable gap 185. These jaws (which may also bereferred to as the “outer width collimator,” or as a “clamshellcollimator”) redefine the width of the beam and enable the final spotwidth to be adjusted if necessary or desired. A hoop 170 rotates inregistration with the inner multi-aperture tube 58. The number of theequally-spaced apertures 175 in hoop 170 is equal to the largest numberof apertures in the rings 58 of tube 50; in this example, there are 24slots 175 spaced 15° apart. The length of the slots 175 is larger thanthe zero-degree slot width of tube 50; that is, the length is greaterthan any of the slots in the inner multi-aperture tube 50. The outerhoop 170 is preferably supported by duplex bearings on the far side.Various elements of the embodiment depicted in FIG. 7 are shownschematically in FIG. 10, for further clarity.

One of various alternate embodiments of the present invention is nowdescribed with reference to FIG. 10A. In what might be referred to as a“bundt aperture system, designated generally by numeral 900,multi-aperture tube 280 and the multi-aperture hoop 290 (of FIG. 9) area single unit 90. Inner apertures 92 and outer apertures 94 co-rotateabout x-ray source 4. Adjustable jaws 16 may be disposed between theco-rotating sets of apertures. The bundt configuration, shown in anassembly view, may not have the versatility of the embodiment depictedin FIG. 7, and it may have a larger rotational moment of inertia, but itdoes have the mechanical advantage of simplicity in changing thesweeping angle, from say 90° to 15°, by step-wise translation of thebundt 90 and its drive motor, which is coupled to shaft 300 (shown inFIG. 7). Different scan angles are selected by translating the bundtscanner so as to register a selected plane of bundt slots with the planeof the fan beam. In accordance with yet another embodiment of thepresent invention, the bundt and drive motor may remain fixed while therest of the unit is translated.

The embodiments described above are but a few of the permutations thatembody the basic concept of an operator-controlled, multi-slotcollimation coupled with a multi-aperture pencil-beam creator. Forexample, the three basic components—width collimator 14, anglecollimator 34 and multi-aperture unit 50—can be permuted in any of thesix possible configurations, the choice being made on the basis ofapplication and mechanical design considerations. One alternateconfiguration would have the x-ray beam traversing unit 34 first, thenunit 14 and finally unit 50. Another has the x-ray beam traverse theunit 50 first, then unit 14 and then unit 34. Similarly, the beam maytraverse the aperture ring 170 and then the variable collimator 180.

It should be noted that among the variations that retain the fundamentalconcepts of zooming with variable beam resolution is the reliance of thevariable angle collimator 34 to act also as the first width collimator,thus eliminating the separate width collimator 14. This simplificationcomes at a cost of some versatility (e.g. the number of opening anglesare more restrictive) but may be useful for some applications, inparticular when using the outer tube configurations of FIG. 7 or FIG.10B in which the width of the beam at the target is controlled by thevariable gap 180 in FIG. 7 or 16 in FIG. 10B.

Filter wheel 150 may provide a variable filter to change the radiationdose delivered to the target or to modify the energy distribution of thex-ray beam. Filters may also be incorporated in the slots of thevariable angle tube 34 to place filters in the 45°, 30° and 15° slotsthat progressively increase the filtration of the lower energycomponents of the x-ray beam, to reduce the dose without significantlyaffecting the higher energy components of the x-ray beam. It should alsobe noted that filter wheel 150 may be omitted, for example, forapplications in which the inspection is always carried out on inanimateobjects. Additionally, filters may be incorporated into a subset of theslots, such as into alternating slots, for example.

In still another configuration, hoop 50 has a larger number of aperturessuch that multiple apertures are illuminated by fan beam 8, producingtwo pencil beams 70 that sweep in alternation through the target atdifferent angles to obtain a stereoscopic view of the interior. Thisapplication uses a wide fan beam and an appropriate multi-aperture unitand slot collimators.

Improving an Image by Improving the Vertical Resolution of the ScanningPencil Beam.

In the discussion, supra, with reference to FIG. 7, slots 175 ofrotating outer hoop 170 are all the same height, h, as depicted in FIG.11A for one set of slots for the four different scan angles, 90°, 45°,30° and 15°, in the example of a preferred embodiment. However, tochange the height resolution, in accordance with alternate embodimentsof the present invention, the slot heights in the outermost rotatingaperture hoop must be changed, as illustrated by the following threeexamples.

FIG. 11B shows an additional ring 102 of half-height slots added to the15° ring of apertures. The operator can select either the 15° or the15s° lateral position; the latter reducing the height of the beam at thetarget by a factor of two. The width the slots in the aperture hoop hasbeen increased by about 3 mm to accommodate the extra ring of apertures.In a preferred embodiment, 4 rings of apertures are maintained, but theheights of all the slots in the 15° ring are halved. This mode uses halfof the six-fold gain in areal intensity of x-rays on the target,compared to the 90° view, to improve the vertical resolution by a factorof 2.

In another embodiment of the invention, rings of apertures of differentheights are added to the 90° viewing angle. That allows automatedchanges in height resolution as a function of the target distance. Atarget passing at a distance of 5 ft. might be most appropriatelyscanned with the aperture ring that has 1-mm slot heights, while atarget passing at 3 feet might be more appropriately scanned with a0.5-mm resolution. It should be clear that, within the practicalconstraints of weight and size, more than one of the above examples canbe accommodated on a single rotating hoop.

Two Independent Views with Different Vertical Resolutions.

Embodiments of the present invention may also be used to simultaneouslyobtain two (or more) images each with its own vertical resolution. FIG.11C shows a slot pattern for obtaining two separate 15° views. Alternate15° sweeps form one image with a vertical resolution h, and anotherimage with a vertical resolution h/2, or smaller. Improved spatialresolution can be essential for resolving issues of interpretation inthe image.

Dual Energy.

In other embodiments of the present invention, filters may be placed inall, or in a subset of, the slots of one of the arrays of slots, witheither the same or different vertical heights, to change the x-rayenergy distribution impinging on the target. In the slot configurationof FIG. 11C, a filter in the alternate slots of the 15° scan can producea separate view that minimizes the lower-energies that inspect thetarget and thus enhances the image of deeper penetrating radiation. Ifall the slots in the 15° scan have the same height, a filter placed inalternate slots may yield new information, including materialidentification, when the filtered image is compared with the unfilteredenergy image.

The two-view or dual-energy modes are achieved to particular advantagein accordance with the present invention. The aperture hoop 170,rotating at the nominal speed of 3600 rpm, makes a 15° scan every 680microseconds. A target vehicle, moving at the nominal speed of 5 kph,travels ˜1 mm during that scan, which is much smaller than the beam sizeat the nominal target distance of 5 feet. As a consequence, the twoviews will be within 10% of overlap registration. The above calculationindicates that even when no provision is made to change the height ofthe pencil beam, the slots in the beam-resolution defining hoop shouldnot have the same heights. The correct heights will depend on theapplication.

Horizontal Resolution.

For distant targets, where two concentric rotating hoops (50 and 170) ofapertures are employed, the horizontal resolution is determined by theslit width 185 of the outer slot collimator 180. The plates that formthe width collimator are controlled by servo-motors. In a preferredembodiment, the width collimator is in the form of a clamshell whose jawopening is controlled by a single motor near the clamshell's hinge. Thewidth may be controlled by the operator or may be automatically changedas a function, for example, of the relative speed of the inspectionvehicle and the target. For inspection of close targets it may not beuseful or desirable to use the outer hoop 170 and the outer slit 125. Inthat case the horizontal resolution would normally be controlled bychanging the width of the 90° slot 24 of the inner tube 14, though othermethods will be apparent to those familiar with mechanical design. Thewidth of slot 24 for the preferred embodiments is nominally 2 mm wide orless, though any slot width falls within the scope of the presentinvention.

The variable width collimator may also be designed to minimize thenon-uniform intensity of the fan beam across the angular range of thefan. The fan beam from an x-ray tube typically exhibits a roll-off inintensity away from the central axis. For a wide-angle fan beam, withangular extent of 90° or more, the roll-off in intensity from thecentral ray can be 30% or more. In FIGS. 8A and 8B, the variable widthcollimator 180 has a non-uniform gap 185. The gap width increases awayfrom the midpoint. For clarity the gap is exaggerated in the depiction.The shape of the opening can be tailored to the angular distribution ofthe x-rays from the x-ray tube; such data is generally supplied by thetube manufacturer.

Dwell Control.

Prior discussion has concentrated on the aspect of the zoom feature,taught herein, which allows for changing the viewing angle whilepreserving the fluence incident on the inspected target. A concomitantaspect of the zoom feature is that the variation with zoom of the numberof scans per unit time has its own advantages and applications. Whenused without changing the collimation, but especially when combined withthe variable collimator, the inspecting beam can be made to spreadevenly over the target so as to minimize undersampling and oversampling.

Undersampling occurs when the beam moves too quickly to allow resolutionof a pixel as defined by the beam cross section, thereby resulting inmissing information. The combination of variable viewing angle andvariable scans per unit time (or, equivalently, dwell time per pixel) isa powerful way to obtain higher throughput with minimum undersampling.In preferred embodiments of the invention, the highest number of scansper revolution for the desired angle of scan is used, and the collimatoris opened to the largest acceptable spatial resolution.

Oversampling, which is not so serious a problem as under-sampling, canbe traded for better resolution. When transverse motion of the sourcerelative to the target is slow, the collimator slot may be narrowed andthe integration time diminished to provide even sampling with improvedresolution.

Offset Hoop.

U.S. Provisional Application Ser. No. 61/533,407 introduces the conceptof backscatter x-ray inspection (BX) by a scanning pencil beam of x-raysproduced by an electron beam whose the axis is offset from the axis ofrotation of a rotating ring of apertures that forms the scanning beam.Offsetting a source behind the axis of rotation of an aperture hoop hadbeen known. The novel forward-offset concept has inherent advantages, asin the application of x-ray inspection portals, where its effectivenessfor faster scanning at close geometries allows a greater throughput ofinspected vehicles. In accordance with embodiments of the presentinvention, components of angle selection and variable-beam resolutionare added to forward offset scanning to significantly increase thesystem's versatility.

In one embodiment of the present invention, a forward-offset portalsystem that inspects vehicles from both sides and the top, can, on thefly, change the angle rate of scan per revolution, as well as the scanresolution and the radiation exposure, to optimally inspect eithertrucks or cars. An effective portal inspection system of cars and trucksmay use the x-ray backscatter technique (BX) to scan from both sides andfrom the top, as the vehicles pass through. The x-ray beams from thethree BX systems are interleaved to prevent cross talk. That requirementplaces a severe limitation on the speed of the inspected vehicles. Forexample, a standard one-sided BX system that uses a 3-spoke aperturehoop, when applied to a three-sided inspection, limits the truck speedto less than 4 kph. To overcome this limitation, U.S. ProvisionalApplication 61/533,407 teaches offsetting the x-ray tube axis forward ofthe axis of the aperture hoop that forms the pencil beams. The forwardoffset concept allows wide-angle scans of trucks with a six-aperturehoop, and a nine- or even a 12-aperture hoop for scanning smallervehicles.

To increase the versatility of the forward offset concept, embodimentsof the present invention in which the axes of the x-ray tube 4 and ofthe hoop 114 of rotating apertures coincide, as now described withreference to FIG. 12. The scan angle of fan beam 110 is defined by anangle selecting ring 111 that may be a variable slot or a ring with twoor more fixed slots. The outer hoop 114 contains the beam-forming ringsof apertures 115, each ring of apertures matches a given selected scanangle. A wide-angle collimator 113 confines the beam from the angleselector 111 to a single ring 115 of apertures. To co-plane the beam 110from the angle selector with the appropriate ring of apertures, thex-ray tube 4 plus angle selector 111 plus collimator 113 are on amovable positioning platform (not shown), which serves to move thoseelements in a direction into the plane depicted in the cross-sectionalview. The system may include the following components:

-   -   a. an x-ray tube 4, well-shielded by a tube shield 112, where        the x-ray tube is offset from the center of a rotating hoop, and        produces a fan beam 110 of x-rays that emanates approximately        perpendicular to the x-ray tube's beam axis.    -   b. a rotatable angle-selector ring 111, with a variable angular        slot or with selectable angular slots, is coaxial with x-ray        tube 4. The ring 111, typically made predominately of lead, is        impenetrable to the x-rays except for the slots that define the        scan angles available for inspection. To optimize the scanning        of both trucks and cars, there may be two or more slots to        accommodate the different heights of trucks, SUVs and cars. The        closed position of the angle-selector ring, which is the default        position when the power is off, is the x-ray shutter for the        system.    -   c. an outer hoop 114, made of material that effectively blocks        all x-rays except for those that pass through equally-spaced        apertures 116, forms the pencil beams 70 of x-rays. The        rotational axis of the hoop 117 is offset from the axis of the        x-ray tube 4 by a distance D (shown in FIG. 13). The apertures        may be arranged in separate rings. The number of apertures in a        given ring must be commensurate with 360°; e.g. six apertures        spaced 60° apart. Each ring of apertures corresponds to one of        the opening angles in the angle-selector ring.    -   d. a collimator (typically, a clamshell collimator) 180 (shown        in FIGS. 7 and 8A-8B) between the collimator ring and the outer        hoop co-planes the fan beam to the appropriate ring of apertures        for the selected angular scan. The azimuthal opening of the        collimator is fixed to accommodate the widest scan angle. The        axial opening angle of the collimator controls the x-ray beam's        axial resolution as well as the radiation dose on the target.

A separate filter ring 150, coaxial with the x-ray tube, with angularsegments of different absorbers to filter the x-ray beam either tocontrol the radiation dose on the target and/or to control the energyspectrum of the x-rays on the target.

X-ray tube 4, angle selector 111 and clamshell collimator 180 aremounted on a platform that moves, under motor control, to place the fanbeam plane in the plane of the aperture ring appropriate for theselected angle. It should be noted that in some applications therotating outer hoop translates, the other components are stationary.

One of the innovations in this invention is the use of rectangular slotsinstead of round or oval holes for purposes of chopping a beam. As usedherein, the terms “slot,” “aperture,” and “through-hole” may be usedinterchangeably. The chopped beam may be a beam of particles having massor of massless particles, including electromagnetic radiation over aspecified wavelength range. In accordance with various embodiments ofthe present invention, a chopper, such as aperture wheel 170 (shown inFIG. 10B), interrupts a beam of particles 70 characterized by apropagation direction. The chopper has a solid portion, which is anobscuring element substantially opaque to passage of the particles inthe propagation direction. Aperture wheel 170 has one or morethrough-holes 175 (shown in FIG. 10B) in the obscuring element adaptedfor passage through the obscuring element of particles in thepropagation direction. Aperture wheel 170 is spun by an actuator (notshown to interpose the through-holes 175 in the beam on a periodicbasis.

The innovation of rectangular chopper apertures has two independentadvantages over traditional round or oval apertures. First, is itsusefulness in a single ring of apertures. The size of the pencil beamdetermines its spatial resolution or point spread function. Oval orcircular apertures result in fixed resolutions that are difficult tochange precisely. Slot apertures have fixed angular widths but variableaxial lengths (dimension parallel to the beam axis) controlled by theclamshell collimator opening. The size of the beam spot can be preciselycontrolled by the collimator opening and the integration time of thepixel. The second advantage is evident when the outer aperture hoop hastwo or more rings of apertures, i.e., zooming ability, each ring with adifferent number of apertures. Round or oval apertures strongly limitthe ability to vary the size of the pencil beams. The use of slots, asexemplified in FIGS. 11A-11C, has both advantages of manufacturabilityand greater range of the axial width for a given length of slot.

The azimuthal widths of the slots are typically the same across all theslots, although they need not be equal, within the scope of the presentinvention. The slot lengths (parallel to the x-ray tube axis) preferablyhave a pattern that is determined by the opening angles ofangle-selector ring 111. In a 3-angle selector ring, for example, onering has 3 apertures spaced 120° apart; an adjacent ring has 6 aperturesspaced 60° apart; a third adjacent ring has 9 apertures spaced 40°apart. The pattern of slots is: 3 long slots at 0°, 120°, and 240° forthe scan angles that are common to the 3 modes, and 12 short slots forscan angles that are unique to their mode. The slotted pattern has thesignificant advantage over round or oval apertures that the axial extentof the beams can be quantitatively adjusted by the clamshell collimatorto change the beam resolution and/or adjust the radiation dose.

In addition to rectangular through-holes, a chopper in accordance withembodiments of the present invention may also have biconical(“hourglass”) or conical through-holes, as shown, respectively, in FIGS.17A-17B, and 17C-17D, respectively. Such shapes advantageously serve tomaximize beam throughput in the face of lateral beam offset. Inembodiments employing more than one ring of apertures, each ring ofapertures should have its own unique conical angles. Slotted aperturespreferably have different slopes along the slots.

The offset scanner concept described herein may be appliedadvantageously to both forward and backward offsets. For heuristicreasons, the offset scanner is describe herein primarily in terms of anoffset in the forward direction (i.e., toward the target, as might beemployed in portal systems for inspecting large and small vehicles,however it is to be understood that the relative position of the tubeaxis and hoop axis does not limit the scope of the invention as claimed.

FIG. 13 is a schematic drawing depicting the geometry of the system. Thetube is forward-offset by a distance D from the axis of the hoop ofradius R. The fan beam exits to the right; the center of the fan is at0°. Apertures 116 along the rim are equally spaced, i.e., commensuratewith 360°. Six apertures, spaced 60° apart are shown in FIG. 13.Aperture 1 is at an angle θ with respect to the hoop axis, and an angleφ with respect to the tube axis; φ>θ. The relationship between φ, θ, Rand D is given by:

$\begin{matrix}{{\tan \; \varphi} = {\frac{\sin \; \theta}{{\cos \; \theta} - \frac{D}{R}}.}} & (1)\end{matrix}$

In the case of a backward offset, D/R is added to cos θ in thedenominator rather than subtracted.

FIG. 14 is a graph of the scan angle (or, equivalently, opening angle,2φ), as a function of the ratio of the forward offset D to the radius R,for 3, 4, 6, 8, 9, and 12 scans per revolution (s/r) of the aperturetube. The number of scans per revolution (or, s/r values) is determinedby the angular spacings, 20, between apertures, which are 120°, 90°,60°, 45°, 40° and 30° respectively. For large values of D/R, φ canbecome negative, i.e., unphysical. (There are no unphysical values ofD/R for backward offsets.) Within obvious constraints, the offset of theaxis of x-ray tube with respect to the axis of the rotating outer hoopcan be any direction and can have a wide range of values, includingzero.

FIG. 14 shows the scan angles obtainable for aperture rings of 3, 4, 6,9 and 12 apertures. A single revolution of the wheel allows the beam toscan 12 times over an angular range of 90 degrees (D/R=0.7), forexample. That spreads the beam intensity over 1080 degrees of scan inone revolution of the aperture hoop. At 6 times per revolution of 120degrees spreads the beam intensity over 720 degrees of scan in onerevolution. This feature, by itself, is a potent tool for reducingundersampling. Undersampling is an inevitable bottleneck to increasingmaximum vehicle speeds to increase throughput. Until now, higherrotational speeds have been sought in order to achieve higherthroughput, however the present invention advantageously providesrequisite additional fluence without resorting to higher rotationalspeeds.

Different D/R values can be used in a single scanning system. In themost general case, the x-ray tube plus a continuously variable angularselector can be moved both radially and axially to produce acontinuously variable anglular scan. In practice, however, the D/R valueis typically fixed. That still gives the system considerable flexibilityto optimize the x-ray beam flux on the target; i.e., to obtain maximumutilization of the fluence. The following examples illustrate.

Example 1

A portal system that uses BX to scan cars, SUVs and trucks from bothsides, but not from the top.

The fan beam from the x-ray tube is collimated to have an azimuthalextent of 120° and an axial width of ˜2°. The x-ray tube is forwardoffset 24 cm from the center of a 60 cm diameter hoop (D/R=0.4). Whentrucks are inspected, the 4-aperture ring is selected and the beamopening angle selector is set at 120°. (A 120° beam is presently thepractical limit for available x-ray tubes.) When cars/SUVs areinspected, the 8-aperture ring, and 72° slot are selected. Theinterleaving requirement results in 2 scans per revolution from eachside of a truck and 4 scans per revolution from each side of a car.

Example 2

A portal system in which cars/SUVs as well as trucks are scanned by BXsystems from both sides and from the top.

The x-ray tube is forward offset 36 cm from the center of a 60 cmdiameter hoop (D/R=0.6). FIG. 14 shows that a 124° scan can be obtained6 times per revolution and a 70° scan can be obtained 12 times perrevolution of the aperture ring. The 124° is effective for scanningsides of trucks while the 70° scan is sufficient for scanning throughthe top of trucks, and sufficient for use from every side of cars andSUVs. Cars can be scanned 4 times per revolution from all three sides.Trucks can be scanned 2 times per revolution from the sides and 4 timesper revolution from the top.

Example 3

Accommodating higher vehicle speeds and hence greater throughput.

An aperture hoop rotating at 3,000 rpm makes one revolution in 0.02seconds. During that time, a vehicle traveling at 12 kph moves 66 mm Theresulting under-sampling with one scan per revolution at an acceptablebeam resolution results in unacceptable inspections at 12 kph. As aconsequence, speeds through present three-side portal inspections arelimited to ˜4 kph because there is only one sweep per revolution fromeach side, Examples 1 and 2 above show that forward offset allows trucksto be scanned twice as rapidly from the sides and 4 times as rapidlyfrom the top. Cars can be scanned 4 times per revolution from everyside. These additional scans per revolution of the beam-forming wheel,together with an adjustable beam width by means of the clamshellcollimator, allows trucks to be effectively inspected at 12 kph, andcars inspected at still higher speeds.

In accordance with various embodiments of the invention, rings ofindividual round or oval apertures may be used. Slots, however, whenused with the clamshell collimator, are preferable, especially whenmultiple rings are used. FIG. 6 shows the pattern of 12 slots forExample 2 above. 6 of the slots, as 0°, 60°, 120, 180°, 240 and 300° aredouble width slots, while 6 slots at 30°, 90°, 150°, and 210°, 270° and330° are single width slots. Either the 6-aperture ring or the 12aperture ring can be selected; the 12 aperture ring is depicted in thefigure. The clamshell collimator can change the aperture width fromfully closed to 5 mm wide in that example, to cover a wide range of beamsizes and delivered dose.

Rotation of the X-Ray Tube.

In accordance with further embodiments of the present invention,provision is made for rotation of x-ray tube 4 about its axis 6 (shownin FIG. 1). Rotatability of the x-ray tube may advantageously increasethe angular volume subject to inspection by the system, and mayadditionally be used to improve the beam resolution, as now describedwith reference to FIGS. 15A-15C, and 16.

As shown in the perspective view of FIG. 16, x-ray tube 4 together withthe angle selector 113, filter ring 150, and clamshell collimator 180are rotatably mounted on a platform 5 that moves linearly to co-planethe selected fan beam with the appropriate ring of apertures. The fanbeam 8 with an angular extent 15, typically provided by the tube'smanufacturer, constrains the ability to change the usable direction andextent of that beam. For example, in the standard configuration in whichthe 120° fan beam from the x-ray tube is emitted horizontally, the basicscanning apparatus 2 can only manipulate the x-ray beam within thatspace. Advantages of a rotatable platform to versatile scanning systemsin accordance with the present invention are now described.

An important application of the rotatable platform is to increase theangular range of backscatter inspection. For example, the maximum heightthat can be inspected in conventional portal systems using a 120° fanbeams is about 14 feet. Higher vehicles cannot be fully inspected. Theaddition of a rotatable platform corrects that problem, allowing asecond inspection of the top portion of a vehicle or targets that are 20feet high or more.

Another important application is to improve the spatial resolution of asecondary inspection of a small area of a vehicle. For example, asuspect area, found in a 120° scan, can be closely inspected by zoominginto the suspect area with a 15° scan. The nine-fold gain in fluxdensity will significantly improve the image of a suspect area. If,however, the suspect region is in the outer reaches of the 120° fan beamfrom the x-ray tube, the spatial resolution of the beam will be far fromoptimum and the full advantage of the zoom will not be realized. Theresolution can be improved substantially by rotating the platform sothat the axial ray of the scanning beam is centered on the suspectregion. The sequence of steps is shown schematically in FIG. 15A to 15C,for a suspect region at the extreme of a 120° scan. In FIG. 15A, the 15°scan, defined by the scan-angle selector 113, is centered on the beamaxis of the 120° fan beam 7 from the tube. The pencil beam emanates froma small, symmetric focal spot and the quality of the pencil beam is thebest it can be for that x-ray tube. Without a rotatable platform, thesuspect area is inspected with a 15° scan by rotating the two arms ofthe scan-angle selector 113 counter-clockwise 52.5°, using actuators 9,to the configuration shown in FIG. 15B. The quality of the pencil beams,however, has worsened because the effective focal spot has grownsubstantially. FIG. 15C shows the same geometry for a 15° scan of thesuspect area, now formed by rotating the platform counter-clockwise52.5° the beam axis from the x-ray tube is along the center of the 15°scan, and the beam quality has been optimized.

Improvement in resolution due to centering the inspected object in thex-ray tube emission beam can be further understood as follows. Thespatial resolution of the backscatter image is determined by thecross-section of the x-ray beam, and that size is constrained by thefocal spot size of the electrons on the anode. The typical x-ray tube(operated in a reflection configuration) focuses a line source ofelectrons (from a coil filament) as a line onto the anode, which istilted with respect to the electron beam. The effective size of thefocal spot depends on the viewing angle. For example, a line source ofx-rays from an anode, tilted 15° with respect to the electron beam, is 1mm high by 4 mm. The line source of electrons spreads the heat load onthe anode, allowing for higher power dissipation and hence higher x-rayflux. The focal spot size of commercial x-ray tubes is specified onlyfor the axial ray direction; in this example, the width of the focalspot is 1 mm and the effective height is also ˜1 mm. The focal spot sizeat the extreme of a 120° fan beam, however, is a line source 1 mm wideby 4×sin 60°=3.5 mm long. Moreover, the beam quality is furtherdiminished by the increased absorption of the x-rays in the anodeitself, the so-called heel effect. Rotating the axial ray from the x-raytube into the center of the zoom angle effectively eliminates both theseeffects.

Degradation of resolution with angular displacement from the center ofthe scan constrains the acceptable angular spread of the scanning pencilbeam. Given that constraint, it is nonetheless often important to obtainthe best spatial resolution for inspecting a specific target area thatis not close to the central axis. To solve this problem the x-ray tubemay be rotated together with the beam collimation so that the centralaxis of the x-ray beam is pointing in the direction of the desiredtarget area.

Operator and Automated Features.

It is to be understood that the focusing operation may be performed byan operator, on the basis of an indicated suspect area that constitutesa portion of the inspected object. The angular opening of the scan, thedirection of the scan, the beam's spatial resolution, and the number ofscans per revolution can each or in combination be changed by theoperator or by automation on the basis of the target height, and targetdistance from the beam chopper assembly, and relative speed of thetarget with respect to the assembly. The identical apparatus may thusadvantageously be employed for performing a primary rapid scan, followedby a secondary, high-resolution, small-area scan of a suspect area foundin a first, rapid scan.

For illustration, the operator may focus on a small, suspect area of atarget that has first been scanned with a broad beam. A 3-aperture ringmay produce a 120° wide scan of a large vehicle. The collimators of theangle selector may then be closed to form a horizontal 15° fan beam,with good resolution since its source is 1 mm×1 mm, in this example. Thecollimators may be rotated together through 52.5° to center the 15° fanbeam onto a specified portion of the inspection target. The x-ray beamis now more concentrated by a factor of 6 compared to the 120° beam, butthe effective source size is now close to 1 mm×3.5 mm and much of theconcentration gain has been lost. The tube/collimator may be rotated sothat the central axis of the beam points along the center of the 15°sweep. The inspection is now carried out with optimum resolution.

The rotation of the x-ray tube reduces the degradation of beamresolution at angles far from the axial direction. In some applicationsit may be advantageous to accept the degradation in resolution andincrease the beam width to obtain as much fluence as possible with thatresolution. One method for doing so is to make the gap 185 of clamshellcollimator 180 in an hour-glass shape, as shown in FIGS. 8A-8B, with theminimum opening of, for example, 3 mm in the center (for the centralrays), increasing as a function of angle to either side of center. Itshould be borne in mind that maximizing the throughput does not equalizethe flux across the scan angle. When the finite size of the focal spotis taken into account, the intensity of the x-ray beam may vary by asmuch as a factor of 2 between its center and the extremes of +−55degrees. The shaped collimator gap serves to equalize flux across thescan angle.

The embodiments of the invention described herein are intended to bemerely exemplary; variations and modifications will be apparent to thoseskilled in the art. All such variations and modifications are intendedto be within the scope of the present invention as defined in anyappended claims. In particular, single device features may fulfill therequirements of separately recited elements of a claim.

1. A scanning apparatus for scanning a beam in a single dimensionalscan, the apparatus comprising: a. a source of radiation for generatinga fan beam of radiation effectively emanating from a source axis andcharacterized by a width; b. an angle selector, stationary during thecourse of scanning, for limiting the angular extent of the scan; and c.a multi-aperture unit rotatable about a central axis in such a mannerthat beam fluence incident on a target is conserved for different fieldsof view of the beam on the target.
 2. A scanning apparatus in accordancewith claim 1, wherein the multi-aperture unit includes rings ofapertures spaced laterally along the central axis in such a manner thatrelative axial motion of the multi-aperture unit, relative to the x-raybeam plane, places a ring of apertures in the beam that is collimated bya corresponding opening angle in the angle selector.
 3. A scanningapparatus in accordance with claim 1, wherein the angle selectorincludes a slot of continuously variable opening.
 4. A scanningapparatus in accordance with claim 1, wherein the central axis issubstantially coincident with the source axis.
 5. A scanning apparatusin accordance with claim 1, wherein the multi-aperture unit includesrectangular slots.
 6. A scanning apparatus in accordance with claim 1,wherein the source axis is forward-offset relative to the central axis.7. A scanning apparatus in accordance with claim 1, further comprising acollimator for limiting at least one of a width of the beam and anangular extent of the scan.
 8. A scanning apparatus in accordance withclaim 7, wherein the collimator is a width collimator.
 9. A scanningapparatus in accordance with claim 8, wherein the collimator is an innercollimator.
 10. A scanning apparatus in accordance with claim 8, whereinthe collimator is an outer collimator.
 11. A scanning apparatus inaccordance with claim 1, wherein the angle selector includes a pluralityof discrete slots.
 12. A scanning apparatus in accordance with claim 10,wherein the angle selector includes a shutter position.
 13. A scanningapparatus in accordance with claim 1, wherein the source of radiation isan x-ray tube.
 14. A scanning apparatus in accordance with claim 1,wherein the source of radiation is of a type generating a fan beamexceeding 60° in opening angle.
 15. A scanning apparatus in accordancewith claim 1, further comprising an enclosing conveyance for conveyingthe scanning apparatus past an inspection target.
 16. A scanningapparatus in accordance with claim 1, wherein the scanning apparatus iscoupled to a platform.
 17. A scanning apparatus in accordance with claim1, wherein the scanning apparatus is coupled to a platform inconjunction with at least one further scanning apparatus.
 18. A scanningapparatus in accordance with claim 1, further comprising a filterdisposed within the beam for changing the energy distribution of thebeam.
 19. A scanning apparatus in accordance with claim 17, wherein thefilter is disposed on a filter tube adapted for selective insertion of aplurality of filters.
 20. A scanning apparatus in accordance with claim18, wherein the filter tube includes a beam shutter.
 21. A scanningapparatus in accordance with claim 1, wherein the multi-aperture unitincludes two nested, multi-aperture collimators.
 22. A scanningapparatus in accordance with claim 1, wherein the multi-aperture unitincludes an inner multi-aperture hoop characterized by a hoop axis, theinner multi-aperture hoop made of material opaque to the beam.
 23. Ascanning apparatus in accordance with claim 21, wherein the innermulti-aperture unit includes rings of apertures spaced laterally alongthe hoop axis in such a manner that relative axial motion of the innermulti-aperture unit, relative to the beam plane, places a ring ofapertures in the beam that is collimated by a corresponding openingangle in the angle selector.
 24. A scanning apparatus in accordance withclaim 1, wherein the multi-aperture unit includes an outermulti-aperture hoop rotatable in registration with an innermulti-aperture hoop.
 25. A scanning apparatus in accordance with claim23, wherein the outer multi-aperture hoop includes a plurality ofapertures configured as horizontal slots in such a manner as to define aminimum size of emitted pencil beams along a sweep direction of thebeams.
 26. A scanning apparatus in accordance with claim 23, wherein theinner multi-aperture hoop and the outer multi-aperture hoop aremechanically integral.
 27. A scanning apparatus in accordance with claim23, further comprising an outer variable width collimator for defining awidth of the beam that enters or exits the outer multi-aperture hoop.28. A scanning apparatus in accordance with claim 1, wherein radiationis emitted through a plurality of apertures at different angles withrespect to the target, such that pencil beams of penetrating radiationsweep in alternation through the target in such a manner as to provide astereoscopic view of an interior volume of the target.
 29. A scanningapparatus in accordance with claim 1, further comprising a rotationassembly adapted to provide for rotation of the source of radiationabout the source axis.
 30. A scanning apparatus in accordance with claim1, wherein the multi-aperture unit includes substantially rectangularthrough-holes.
 31. A chopper for interrupting a beam of particles, thebeam of particles characterized by a propagation direction, the choppercomprising: a. an obscuring element substantially opaque to passage ofthe particles in the propagation direction; b. at least one through-holein the obscuring element adapted for passage through the obscuringelement of particles in the propagation direction, the through-holecharacterized by a tapered dimension in a plane transverse to thepropagation direction; c. an actuator for moving the obscuring elementin such a manner as to cause at least a portion of the beam of particlesto traverse the at least one through hole on a periodic basis.
 32. Achopper in accordance with claim 31, wherein the through-hole issubstantially conical.
 33. A chopper in accordance with claim 31,wherein the through-hole is substantially biconical.
 34. A chopper inaccordance with claim 31, wherein the particles are massless.
 35. Achopper in accordance with claim 31, wherein the obscuring element isopaque to passage of electromagnetic radiation over a specified range ofwavelengths.
 36. A chopper for interrupting a beam of particles, thebeam of particles characterized by a propagation direction, the choppercomprising: a. an obscuring element substantially opaque to passage ofthe particles in the propagation direction; b. at least one through-holein the obscuring element adapted for passage through the obscuringelement of particles in the propagation direction, the through-holecharacterized by a rectangular cross section in a plane transverse tothe propagation direction.
 37. A chopper in accordance with claim 36,wherein the through-hole is disposed within a rotating element.
 38. Acollimator for narrowing a beam of particles, the beam of particlescharacterized by a propagation direction, the collimator comprising: a.an obscuring element substantially opaque to passage of the particles inthe propagation direction; b. a gap in the obscuring element adapted forpassage through the obscuring element of particles in the propagationdirection, the gap characterized by a length taken along a longdimension and a width taken along narrow dimension, both the long andnarrow dimensions transverse to the propagation direction, wherein thewidth of the gap varies as a function of distance along the longdimension relative to an edge of the gap.
 39. A collimator in accordancewith claim 38, wherein the gap is fixed.
 40. A collimator in accordancewith claim 38, wherein the gap is adapted to be varied in at least oneof the long and narrow dimensions.
 41. A method for inspecting an objectbased on the transmission of x-rays through an object, the methodcomprising: a. generating a fan beam of radiation; b. collimating thewidth of the fan beam with a collimator that is stationary during thecourse of a scan of the object, the scan characterized by an extent; c.limiting the extent of the scan with an angle selector; and d. varying afield of view of the beam on the object by means of a multi-apertureunit rotatable about a central axis in such a manner that the resultingbeam fluence on the target is the same per revolution for all selectedscan angles.
 42. A method in accordance with claim 41, furthercomprising varying a direction of the scan collimator.
 43. A method inaccordance with claim 41, wherein the step of varying a field of viewincludes varying from a primary rapid scan to a secondaryhigh-resolution scan of a suspect area.